In the treatment of substrates, for example, coating or diffusion treatment of flat substrates to produce coated glass panes or thin-layer solar cells, the substrates must be heated. Surface heating devices are known for this purpose in which a so-called jacket tube heater is used.
Jacket tube heaters have a resistance heating wire in a jacket tube made of aluminum, stainless steel or other metallic material, which is electrically insulated relative to the jacket tube, for example, embedded in ceramic insulation elements made of aluminum oxide or other appropriate substances.
It is important to achieve the most uniform possible heating of the substrate. Surface heating devices in which one or more jacket tube heaters are shaped so that straight and bent tube sections alternate, the straight tube sections running parallel to each other in a plane, referred to subsequently as main plane, are therefore used in so-called continuous installations for treatment of wafer-like substrates. Each two adjacent straight tube sections are connected to each other on the edge of the surface heating device by a bent tube section, which also lies in the main plane. The surface heating device is arranged in the substrate treatment device generally so that the main plane is aligned parallel to the transport plane of the substrates, the straight tube sections of the jacket tube heater generally being aligned across the transport direction of substrates.
A surface heating device is to be understood subsequently to mean a heating device in which three or more straight tube sections, which are electrically connected in series and are electrically heatable by common current connections, are arranged in the same plane subsequently called the main plane and in which each two adjacent straight tube sections are connected to each other by a bent tube section. Owing to the fact that the straight tube sections are arranged parallel to each other in the same plane, such a surface heating device radiates the heat energy generated by it mostly perpendicular to the main plane.
This is a critical difference relative to heating devices that generate heat energy essentially point-like and therefore radiate uniformly to all sides, like the device described in JP 09236268 A, or radiate the heat along a circular line, like the device known from U.S. Pat. No. 2,483,839.
On the other hand, a heating element is known from U.S. Pat. No. 4,079,921 in which a straight tube section lying in the main plane is connected to two straight tube sections lying in a secondary plane by bent tube sections, but each straight tube section lying in the main plane here has its own power supply and only by corresponding arrangement of numerous such heating elements is a surface device produced in which the numerous straight tube sections lying in the same main plane form a surface heating device. A shortcoming in this solution is, for example, that numerous current connections are required, but also that the straight tube sections lying in the secondary plane are unavoidable, regardless of whether their heating power is required or not.
On the other hand, the invention proceeds from a surface heating device in which the three or more straight tube sections generally of essentially the same length, which are electrically heatable by a common power supply, are arranged in the same plane, subsequently called the main plane, and in which each two adjacent straight tube sections are connected to each other by a bent tube section. In other words, the heating wires or sections of a heating wire that are passed through the straight tube sections are connected electrically in series and are fed by common current connections that are arranged on each end of the configuration of straight and bent tube sections.
To produce such surface heating devices the jacket tube heater must therefore be bent repeatedly in order to obtain the bent tube sections, which connect the straight tube sections to each other. There is a restriction in that the bending radii of the bent tube sections cannot be chosen arbitrarily small. Depending on the diameter, wall thickness and material of the jacket tube, the bending radius must not be smaller than the stipulated minimum radius. This results in the difficulty that the straight tube sections cannot be arranged arbitrarily close to each other, for which reason the power density of the surface heating device has limits. The homogeneity of heat input into the substrate underneath also suffers in that the straight tube sections have a minimum spacing dictated by the minimum radius.
The invention is supposed to create a remedy here in which the power density is increased and the homogeneity of heat radiation improved.